One of Christchurch's worst serial rapists has for the first time confessed to an attack he was DNA linked to, but never charged with.

Tavita Tuetue, now 57, has also taken full responsibility for his four other convictions, admitting he "dragged his victims through hell".

As a result of his latest confession, made to the Parole Board this month, police are reviewing the unresolved case.

However, they say it is unlikely charges will be laid as the victim did not wish to make a formal complaint.

Tuetue was sentenced to preventive detention in 2000 for four night-time home invasions and sex attacks on young women in Christchurch in 1980, 1988, 1994 and 1996. Two women were raped and two managed to fight back until he fled.

Tuetue was linked to two of the attacks through DNA matching years after they occurred. He was the first offender in New Zealand to be convicted this way.

Recent Parole Board reports showed Tuetue had taken steps that could allow his eventual release. He had completed a sex offender treatment programme and was going on escorted outings to go shopping and visit a friend.

Then in February last year, DNA evidence linked Tuetue to an unsolved 1993 sex crime.

Police shelved the file after the victim decided against prosecution.

Tuetue also maintained his innocence.

According to the lastest Parole Board report, Tuetue's about-face confession to the allegation was made without prompting.

"He said he had not been prepared to discuss it before but that, upon [a supporter's] death and upon reflection, he wanted to acknowledge full responsibility for all his offending," the report said.

"He said he should have done the same thing in the past in respect of his other offending, but, instead, he dragged his victims through hell."

The Parole Board asked Tuetue if he had received legal advice in respect of the new admission, as it put him at "obvious risk of prosecution and a further finite sentence".

Tuetue replied he had not, and did not want it.

"However, we doubt that he appreciated the full consequences of doing so," the board said.

Police were unaware of the admission until alerted by The Press and, as a result, said the file would be reviewed.

Detective Inspector Greg Murton said police may yet interview Tuetue about his statement to the parole board. However, they remained mindful that the victim wanted no prosecution when spoken to last year. Investigators were deciding what action, if any, would be taken.

Even if a prosecution could proceed without a victim giving evidence, it was "extremely unlikely" to happen because of the potential a court case could cause her more trauma.